chapter 8 an introduction to microbial metabolismclasspages.warnerpacific.edu/sramos/bio370...
TRANSCRIPT
Chapter 8
An Introduction to Microbial Metabolism
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The Metabolism of Microbes
Metabolism – all chemical and physical workings of a cell
Two types of chemical reactions:
Catabolism – degradative; breaks the bonds of larger molecules forming smaller molecules; releases energy
Anabolism – biosynthesis; process that forms larger macromolecules from smaller molecules; requires energy input
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Enzymes
• Enzymes are biological catalysts that increase the rate of a chemical reaction by lowering the energy of activation
• The energy of activation is the resistance to a reaction
• The enzyme is not permanently altered in the reaction
• Enzyme promotes a reaction by serving as a physical site for specific substrate molecules to position
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6
Enzyme Structure
• Simple enzymes – consist of protein alone
• Conjugated enzymes or holoenzymes –
contain protein and nonprotein molecules
– Apoenzyme – protein portion
– Cofactors – nonprotein portion
• Metallic cofactors: iron, copper, magnesium
• Coenzymes, organic molecules: vitamins
Conjugated enzyme structure
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Apoenzymes: Specificity and the
Active Site
• Exhibits primary, secondary, tertiary, and some, quaternary structure
• Site for substrate binding is active site, or catalytic site
• A temporary enzyme-substrate union occurs when substrate moves into active site –induced fit
• Appropriate reaction occurs; product is formed and released
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Carrier
functions of
coenzymes
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Location and Regularity of
Enzyme Action
• Exoenzymes – transported extracellularly,
where they break down large food
molecules or harmful chemicals
– Cellulase, amylase, penicillinase
• Endoenzymes – retained intracellularly and
function there
– Most enzymes are endoenzymes
Types of enzymes
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• Constitutive enzymes – always present, always produced in equal amounts or at equal rates, regardless of amount of substrate
– Enzymes involved in glucose metabolism
• Regulated enzymes – not constantly present; production is turned on (induced) or turned off (repressed) in response to changes in concentration of the substrate
Constitutive and regulated enzymes
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Synthesis and Hydrolysis Reactions
• Synthesis or condensation reactions –
anabolic reactions to form covalent bonds
between smaller substrate molecules,
require ATP, release one molecule of water
for each bond formed
• Hydrolysis reactions – catabolic reactions
that break down substrates into small
molecules; requires the input of water to
break bonds
Sensitivity of Enzymes to Their
Environment
• Activity of an enzyme is influenced by
cell’s environment
• Enzymes operate under temperature, pH,
and osmotic pressure of organism’s habitat
• When enzymes are subjected to changes in
organism’s habitat they become unstable
– Labile: chemically unstable enzymes
– Denaturation: weak bonds that maintain the
shape of the apoenzyme are broken18
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Regulation of Enzymatic Activity
and Metabolic Pathways
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Direct Controls
on the Actions of Enzymes
1. Competitive inhibition – substance that resembles normal substrate competes with substrate for active site
2. Noncompetitive inhibition – enzymes are regulated by the binding of molecules other than the substrate on the active site
• Enzyme repression – inhibits at the genetic level by controlling synthesis of key enzymes
• Enzyme induction – enzymes are made only when suitable substrates are present
Regulation of enzyme action
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Enzyme repression
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The Pursuit and Utilization of
Energy
• Energy: the capacity to do work or to cause
change
• Forms of energy include
– Thermal, radiant, electrical, mechanical,
atomic, and chemical
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Cell Energetics
• Cells manage energy in the form of chemical reactions
that make or break bonds and transfer electrons
• Endergonic reactions – consume energy
• Exergonic reactions – release energy
• Energy present in chemical bonds of nutrients are
trapped by specialized enzyme systems as the bonds
of the nutrients are broken
• Energy released is temporarily stored in high energy
phosphate molecules. The energy of these molecules
is used in endergonic cell reactions.
Cell Energetics
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X Y Z EnergyEnzyme
+ +
A B CEnergyEnzyme
++
Exergonic
Endergonic
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Biological Oxidation and Reduction
• Redox reactions – always occur in pairs
• There is an electron donor and electron
acceptor which constitute a redox pair
• Process salvages electrons and their energy
• Released energy can be captured to
phosphorylate ADP or another compound
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Electron and Proton Carriers
• Repeatedly accept and release electrons and
hydrogen to facilitate the transfer of redox
energy
• Most carriers are coenzymes:
NAD, FAD, NADP, coenzyme A, and compounds
of the respiratory chain
Details of NAD reduction
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Adenosine Triphosphate: ATP
• Metabolic “currency”
• Three part molecule consisting of:
– Adenine – a nitrogenous base
– Ribose – a 5-carbon sugar
– 3 phosphate groups
• ATP utilization and replenishment is a
constant cycle in active cells
• Removal of the terminal phosphate releases
energy
Structure of ATP
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Phosphorylation of glucose by ATP
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Formation of ATP
ATP can be formed by three different mechanisms:
1. Substrate-level phosphorylation – transfer of
phosphate group from a phosphorylated
compound (substrate) directly to ADP
2. Oxidative phosphorylation – series of redox
reactions occurring during respiratory pathway
3. Photophosphorylation – ATP is formed
utilizing the energy of sunlight
Formation of ATP by substrate-level
phosphorylation
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Pathways of Bioenergetics
• Bioenergetics – study of the mechanisms of
cellular energy release
• Includes catabolic and anabolic reactions
• Primary catabolism of fuels (glucose) proceeds
through a series of three coupled pathways:
1. Glycolysis
2. Kreb’s cycle
3. Respiratory chain, electron transport
Major Interconnections of the Pathways
in Aerobic Respiration
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Metabolic Strategies
• Nutrient processing is varied, yet in many cases is
based on three catabolic pathways that convert
glucose to CO2 and gives off energy
• Aerobic respiration – glycolysis, the Kreb’s
cycle, respiratory chain
• Anaerobic respiration – glycolysis, the TCA
cycle, respiratory chain; molecular oxygen is not
final electron acceptor
• Fermentation – glycolysis, organic compounds
are the final electron acceptors
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Aerobic Respiration
• Series or enzyme-catalyzed reactions in which
electrons are transferred from fuel molecules (glucose)
to oxygen as a final electron acceptor
• Glycolysis – glucose (6C) is oxidized and split into 2
molecules of pyruvic acid (3C), NADH is generated
• TCA – processes pyruvic acid and generates 3 CO2
molecules , NADH and FADH2 are generated
• Electron transport chain – accepts electrons from
NADH and FADH; generates energy through
sequential redox reactions called oxidative
phosphorylation
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Electron Transport and Oxidative
Phosphorylation
• Final processing of electrons and hydrogen and
the major generator of ATP
• Chain of redox carriers that receive electrons
from reduced carriers (NADH and FADH2)
• ETS shuttles electrons down the chain, energy
is released and subsequently captured and used
by ATP synthase complexes to produce ATP
– Oxidative phosphorylation
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The Formation of ATP and
Chemiosmosis
• Chemiosmosis – as the electron transport carriers
shuttle electrons, they actively pump hydrogen
ions (protons) across the membrane setting up a
gradient of hydrogen ions – proton motive force
• Hydrogen ions diffuse back through the ATP
synthase complex causing it to rotate, causing a 3-
dimensional change resulting in the production of
ATP
Chemical and Charge Gradient between
the Outer and Inner Compartments
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Electron Transport and ATP Synthesis in Bacterial Cell
Envelope
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The Terminal Step
• Oxygen accepts 2 electrons from the ETS and
then picks up 2 hydrogen ions from the
solution to form a molecule of water. Oxygen
is the final electron acceptor
2H+ + 2e-
+ ½O2 → H2O
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Anaerobic Respiration
• Functions like aerobic respiration except it
utilizes oxygen containing ions, rather than free
oxygen, as the final electron acceptor
– Nitrate (NO3-) and nitrite (NO2
-)
• Most obligate anaerobes use the H+
generated
during glycolysis and the Kreb’s cycle to
reduce some compound other than O2
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Fermentation
• Incomplete oxidation of glucose or other carbohydrates in the absence of oxygen
• Uses organic compounds as terminal electron acceptors
• Yields a small amount of ATP
• Production of ethyl alcohol by yeasts acting on glucose
• Formation of acid, gas, and other products by the action of various bacteria on pyruvic acid
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Final
acceptor is
organic
compound
and there is
regeneration
of NAD
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Products of pyruvate fermentation
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Biosynthesis and the Crossing
Pathways of Metabolism
• Many pathways of metabolism are bi-directional or amphibolic
• Catabolic pathways contain molecular intermediates (metabolites) that can be diverted into anabolic pathways
– Pyruvic acid can be converted into amino acids through amination
– Amino acids can be converted into energy sources through deamination
– Glyceraldehyde-3-phosphate can be converted into precursors for amino acids, carbohydrates, and fats
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Reactions that produce and convert amino
acids
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Photosynthesis: The Earth’s
Lifeline
• The ultimate source of all the chemical
energy in cells comes from the sun
6CO2 + 6H2O C6H12O6 + 6O2
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light
Overview of photosynthesis
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Photosynthesis
• Occurs in 2 stages
• Light-dependent – photons are absorbed by chlorophyll, carotenoid, and phycobilin pigments
– Water split by photolysis, releasing O2 gas and provide electrons to drive photophosphorylation
– Released light energy used to synthesize ATP and NADPH
• Light-independent reaction – dark reactions –Calvin cycle – uses ATP to fix CO2 to ribulose-1,5-bisphosphate and convert it to glucose
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